A Multiple Input Multiple Output (MIMO) antenna system normally encompasses two fundamentally different physical channels, namely the broadcast channel and the shared channel. The broadcast channel transmits information that needs to be received by all user terminals (e.g. User Equipment, UEs) within the intended cell coverage of the Base Station (BS). The shared channel on the other hand is sliced up in time-frequency-space fragments where some user terminals use some fragments and some other user terminals use other fragments.
The synchronization and cell search signals Primary Synchronization Channel (PSCH) and Secondary Synchronization Channel (SSCH), the Physical Broadcast Channel (PBCH), the cell specific Reference Signals (RS), the control signals (PCFICH, PHICH, PDCCH) have the common property that these need to be broadcasted all over the cell that is intended to be covered by the BS.
Further, there are a number of possible transmission modes in 3GPP Long Term Evolution (LTE). The LTE system allows for quite high flexibility in number of logical antenna ports to be used, and how to use mentioned antenna ports. Information about the transmission modes is given in the specifications for LTE.
Moreover, the number of logical antenna ports in a MIMO antenna system is drastically increased compared to the number of logical antenna ports in a conventional BS antenna system. In the conventional antenna system there are only two antenna ports, while it is common to have 8 or even higher number of antenna ports in MIMO BS antenna systems. For a BS with few wide beam antennas it is a simple task to achieve broadcast beamforming, since the mapping of a signal on one antenna port only, the single wide beam antenna inherently generates a wide beam. Therefore, the signals transmitted on this antenna will be heard widely over the cell. The same principle also applies for a MIMO BS antenna system. A signal transmitted on a single antenna will be widely transmitted over the spatial domain. However, when the number of antennas and amplifiers in a MIMO antenna system increases, this also implies that the available power per antenna port decreases, so the widely transmitted beam will have less power the more ports the MIMO system has.